Content area
Full Text
Cohort, cross sectional, and case-control studies are often referred to as observational studies because the investigator simply observes. No interventions are carried out by the investigator. With the recent emphasis on evidence based medicine and the formation of the Cochrane Database of randomised controlled trials, such studies have been somewhat glibly maligned. However, they remain important because many questions can be efficiently answered by these methods and sometimes they are the only methods available.
The objective of most clinical studies is to determine one of the following-prevalence, incidence, cause, prognosis, or effect of treatment; it is therefore useful to remember which type of study is most commonly associated with each objective (table 1)
Objective | Common design |
Prevalence | Cross sectional |
Incidence | Cohort |
Cause (in order of reliability) | Cohort, case-control, cross sectional |
Prognosis | Cohort |
Treatment effect | Controlled trial |
While an appropriate choice of study design is vital, it is not sufficient. The hallmark of good research is the rigor with which it is conducted. A checklist of the key points in any study irrespective of the basic design is given in box 1.
Box 1 Study purpose
The aim of the study should be clearly stated.
Sample
The sample should accurately reflect the population from which it is drawn.
The source of the sample should be stated.
The sampling method should be described and the sample size should be justified.
Entry criteria and exclusions should be stated and justified.
The number of patients lost to follow up should be stated and explanations given.
Control group
The control group should be easily identifiable.
The source of the controls should be explained-are they from the same population as the sample?
Are the controls matched or randomised-to minimise bias and confounding.
Quality of measurements and outcomes
Validity-are the measurements used regarded as valid by other investigators?
Reproducibility-can the results be repeated or is there a reason to suspect they may be a "one off"?
Blinded-were the investigators or subjects aware of their subject/control allocation?
Quality control-has the methodology been rigorously adhered to?
Completeness
Compliance-did all patients comply with the study?
Drop outs-how many failed to complete the study?
Deaths
Missing data-how much are unavailable and why?
Distorting influences
Extraneous treatments-other interventions that may have affected some but not all of...